Kerberos KRB_AP_REQ message - Server name verification required ?

[Benjamin Kaduk]

On Fri, Mar 19, 2021 at 11:47:49PM +0530, Vipul Mehta wrote:
> Hi,
> 
> Suppose there are two servers A and B running under different kerberos
> service principals.
> If both the service principals have same password and kvno then kerberos
> long term encryption key will be same for both. Seems to be the case for
> windows KDC.
> 
> In such case, a client having service ticket for A tries to authenticate
> with that ticket with server B, should it work ? It is working fine in JDK
> implementation.
> 
> https://tools.ietf.org/html/rfc1510#page-21 : in RFC it is not clear
> whether server should validate server principal in the service ticket when
> KRB_AP_REQ message is received. Looks like just decryption with key is
> sufficient along with some other validations but i don't find server name
> validation explicitly mentioned.

I note that RFC 1510 has been obsoleted by RFC 4120 (but
https://tools.ietf.org/html/rfc4120#section-3.2.3 contains essentially the
same text that you reference).

My understanding is that the RFC authors assumed that different services
would have different keys, so the scenario you describe would not occur
(though, as you know, the situation does occur quite often in Active
Directory environments).  Since the Ticket sname is in the unencrypted part
of the ticket, there is no value in validating its contents, as the Ticket
could be re-encoded with an arbitrary sname value.  It is, in essence, just
a hint for locating the proper key, in the same that the realm is (and the
realm is explicitly discussed as serving this role in the referenced text).

-Ben